Abstract

The ordinary 12-oz beverage cans in the figures below are not held up with any props or glue. The bottom of such cans is stepped at its circumference for better stacking. When this kind of can is tilted, as shown in Fig. 1, the outside corners of the step touch the surface beneath, providing an effective contact about 1 cm wide. Because the contact is relatively wide and the geometry is symmetrical, it is easy to balance an empty can by simply adding an appropriate amount of water so that the overall center of mass is located directly above the contact. In fact, any amount of water between about 40 and 210 mL will work. A computational animation of this trick by Sijia Liang and Bruce Atwood that shows center of mass as a function of amount of added water is available at http://demonstrations.wolfram.com. Once there, search “balancing can.”